Analysis of satellite imagery taken after a deadly attack on an aid convoy in northern Syria last month shows that it was an air strike, a UN expert says.
At least 18 people were killed when lorries unloading at a Syrian Arab Red Crescent warehouse in a rebel-held town outside Aleppo came under fire.
The US believes Russian warplanes bombed the convoy. Russia, which backs Syria’s government, denies the charge.
UN officials have said the incident could constitute a war crime.
Secretary General Ban Ki-moon has established an internal board of inquiry to investigate the attack, which he has denounced as “sickening, savage and apparently deliberate”.
The attack came days after a US-led coalition strike in the eastern city of Deir al-Zour, which killed at least 62 Syrian government soldiers.
A week-long ceasefire brokered by Russia and the US collapsed soon after the two attacks, and the Syrian army, backed by Russia, launched a fresh offensive on eastern Aleppo where 275,000 people are currently trapped.
The intensity of the bombing has since increased with air strikes – some allegedly involving bunker-busting, incendiary and cluster munitions – killing at least 420 people and injuring more than 1,000 others.
The area was officially declared a “besieged area” on Wednesday, the spokesman for the UN’s humanitarian agency OCHA told the AFP news agency.
It met the UN’s three criteria of military encirclement, lack of humanitarian access and no freedom of movement for civilians, Jens Laerke said.